Explosive release ignition assembly



June 15, 1965 w. 1.. DAHL 3, 8

EXPLOSIVE RELEASE IGNITION ASSEMBLY Filed Aug. 30, 1963 2 Sheets-Sheet 1INVENTOR. WALTER L. DAH

ATTORNEY Jun 15, 1965 w. L. DAHL 3,188,914

EXPLOSIVE RELEASE IGNITION ASSEMBLY Filed Aug. 30, 1963 2 Sheets-Sheet 2INVENTOR. WALTER L. DAH L ATTORNEY rates nite This invention relates toan electrical ignition assembly that is particularly well adapted foruse in electric intiators meeting the exacting requirements inminiaturization of explosive devices.

With the growing need for miniaturization in commercial and militaryendeavors, requirements have been set up for miniature electricinitiators having specific characteristics of physical design and firingperformance. For example, a current military specification calls for anelectric initiator having an outside diameter not larger than 0.09 inchand a length not greater than A inch. The initiator should function overa wide temperature range in response to a specific firing energy in therange of about 5000 to about 20,000 ergs supplied from a small capacitorat voltages of from 40 to 100 volts with a firing time not to exceedabout 300 microseconds. The initiator should be capable of withstandingrough handling and acceleration. Even though it would be desirable tomake even smaller units with even more stringent firing characteristics,the standards set forth are difficult to meet satisfactorily on a massproduction basis by the types of initiators heretofore available,primarily due to the need for a suitable ignition assembly.

Presently available electric initiators conventionally employ anignition assembly comprising a bridgewire spanning two essentiallyparallel lead wires, i.e., electrodes or conductors, extending into theshell from an external firing circuit and held in place by a resilient,insulating plug. The bridgewires, in the preponderance of cases, arespaced from the base of the plug and surrounded by a bead of ignitioncomposition. One distinct drawback to the miniaturization of this typeof ignition assembly is that attempts to reduce the diameter and size ofthe bridgewires or lead wires to provide smaller units responsive tolower firing energies results in units where the fine wires extendingfrom the plug into the shell are vulnerable to breakage duringfabrication and assembly of the initiator and are not readily solderedto provide electrical connections which function after handling andstorage. In small sizes, such assemblies are extremely sensitive todouble lead wire to shell static discharge. When used in explosivefasteners, bridge plugs of this variety also require excessive lengthsof sealing compound behind the plug if leal -proof functioning is to beachieved.

Another type of ignition assembly heretofore used, particularly formilitary devices, employs a graphite film bridge. This type of assemblyis generally formed by twisting together two pieces of insulated leadwire, molding the twisted Wires into an insulating plastic block, andcutting the block to expose two adjacent wire ends. The two exposed wireends are covered with a graphite or metaliic film, then with at leastone layer of a primary explosive composition in a lacquer-like binder.The graphite or metalic film bridge is subject to deterioration over aperiod of time and is subject to accidental actuation by impact.

Representative of still another approach to providing an ignitionassembly which will meet the stringent requirements for a small unitgenerally involves the use of a beaded or rivet-shaped center electrodereceived in a generally cylindrical insulator and a sleeve-like outputelectrode which serves as a housing for a conductive ignition charge.Such an ignition assembly is subject to wild and atent erratic variationin firing characteristics since it is exceedingly diificult to maintainthe homogeneous mixture of ignition composition and conductive material,e.g., graphite, amorphous carbon, metal particles, etc., that isrequisite for uniform, reliable firing of the assembly. This difficultyis magnified when the units are to be stored since agglomeration of theconductive particles is likely to increase markedly with time and theaddition of any dispersing or stabilizing agent to the mix is made withthe risk of radically altering the firing characteristics of the mix.Further, the assemblies of this type are susceptible to accidentalactuation by stray current, static charges, and electromagneticradiation. 7

The disadvantages of the prior art are avoided by the novel ignitionassembly of this invention which is suitable for production in smallsizes with high standards of uniformity and reliability and isespecially useful in assembling explosive fasteners that are sealed bothbefore and after actuation. The ignition assembly comprises:

(1) A plug of an electrical insulating material comprising (a) a bodyelement of essentially cylindrical crosssection having a head portionand a smaller diameter portion, a cavity having an essentiallycylindrical wall being formed in said smaller diameter portion, saidcavity being concentric with the longitudinal axis of said body element,(b) a washer element of essentially truncated conical longitudinalcross-section contiguous to the base of said head portion and inperipheral engagement with the smaller diameter portion of said bodyelement, and (c) an internal electrode-receiving zone bounded by saidbase, the periphery of said smaller diameter portion, and the bore ofsaid washer,

(2) A first electrode extending through the head portion of the bodyelement and terminating in said electrodereceiving zone,

(3) A ground-contact electrode spaced from said first electrode andextending from said zone through the head portion of said body element,the extremity of said groundcontact electrode within said zone beingspaced from said cavity and spaced from the end of the first electrodein said zone by a distance at least equal to the radius of the smallerdiameter portion of said body element,

(4) An electrical element of high resistance bridging the extremities ofsaid electrodes within said zone and passing through said cavity and thesmaller diameter portion of said body element, and

(5) A heat-sensitive ignition composition within said cavity and incontact with said bridgewire.

When the ignition assembly of this invention is to be used in anexplosive fastener, the axis of the first electrode at its point ofentry into the head portion of the body element will be essentiallycoincident with the plugs longitudinal axis and said electrode will havean integral inclined portion extending radially from said axis anddefining an angle of declination with respect thereto, the inclinedportion beginning within the head portion and terminating in theelectrode-receiving zone. In this case, the ground contact electrodewill be essentially parallel to but radially displaced from the plugslongitudinal axis.

As used herein, the expression essentially means for all intents andpurposes and is not intended to denote absolute necessity. For example,an essentially cylindrical element is either a cylindrical element orone which is substantially cylindrical and also functionally equivalentto the cylindrical element.

The phrase an internal electrode-receiving zone bounded by said base,the periphery of said smaller diameter portion, and the bore of saidwasher is intended to require that there be a passage or pockets atleast partially surrounding the smaller diameter portion of the bodyelement and communicating with the base of the head por- 3 tion toaccommodate the extremities of the first and second electrodes whichextend from said base. Thus, the washer encloses the electrodeextremities. For example, the bore of the washer adjacent to the base ofthe head portion can be enlarged to provide an annular passage undersaid base, or the bore can contain separate pockets for the respectiveelectrode extremities separated, e.g., by a distance at least equal tothe radius of the plugs smaller diameter portion. For ease of assembly,the. bore of the washer can be coated with an electrically nonconductiveresin before being pressed over the plug body. The use of thenonconductive resin in conjunction with the plug and washer provides anassembly in which the electrical elements are securely imbedded andresistant to the effects of shock or vibration.

In an embodiment of this invention preferred for use in initiators ofsmall size, e.g., having detonator shells about 0.090 inch in diameterand 0.375 inch in length, the portions of the electrodes which join thebridgewire are 'wrapped around the plug subtending an arc of from about30 to about 60 and the electrical element of high resistance extendsthrough the plug at an angle of about 90 to the axis of the plug and issecured under the electrodes. The ignition assembly may be used toinitiate the explosive train of detonators, squibs, or other suchexplosive i to about 50 milligrams. In the case of a mild steelfastener, the bore in the housing is about A to about /2 the diameter ofthe fastener at the section of the fastener at which release is tooccur. The length of the bore in the fastener may be about inch to about1 inch and the diameter of the bore is about inch to about /3 inch.

In order to describe the invention in greater detail, reference is nowmade to the accompanying drawings in which: 7

FIGURE 1 is a cross-sectional view of one embodiment of anexplosively-releasable fastener having an ignition assembly inaccordance with this invention, greatly device. 'Since the ignitionassembly is well adapted to manufacture in small sizes, the assembly isparticularly useful in the miniature initiators in which the explosivetrain may comprise a base charge of from about 10 to about 50 milligramsof a high velocity detonating explosive'a'nd a priming charge of fromabout 2 to about milligrams of a primary explosive. The shell for suchan initiator is only about 0.09 inch in diameter and /s inch long, andmay have a bottom coined or. thinned in comparison with the side wallsso that the effective energy produced by the base charge is allowed toachieve optimum initiation impulse, i.e., to focus the initiationimpulse produced along the longitudinal axis of the initiator.

The ignition assemblies of this device wherein the insulating materialis a rigid materialv such as an epoxy resin, a sintered ceramic,Bakelite, a refractory material, or the like, are particularly welladapted for use in an explosively releasable fastener since the physicalconfiguration of the plug enables the ignition assembly to function as amechanical seal in conjunction with a rigid member reducing the bore inthe fastener above the plug and eliminates the straight-line path fordetonation gases from the bore of the fastener containing the explosivetrain to the exterior of the fastener that exists in common bridgeplugs. Consequently, gases produced upon actuation of the explosivetrain will not be vented or leak from the head of. the fastener beforeor after separation of the fastener. In a preferred explosivelyreleasable fastener, the ignition assembly of this invention isseparated from the priming charge by a distance less than the gapsensitivity value of the priming charge, i.e., the maximum distance orair gap over which the priming charge can be initiated by the stimulusof the ignition composition. The bore of the fastener then preferably isstepped to be of 'smaller diameter in the section between the ignitionassembly and the base of the plug than in the sections a'bout thepriming and base charges. In the latter case, loading of the'bore of thefastener is accomplished from the end of the fastener opposite theelectrical ignition assembly and the loading aperture is sealed with aclosure screw.

7 While it is not desired to be limited by theory, it is believed thatthis narrowing of the bore reduces the magnitude ofthe detonation stressonthe insulating plug and permits a reduction in the thickness or heightof the head portion of the plug for any given insulating material whilestill allowing a leak proof seal to be retained when the fastener issevered. The loading of the base charge in such a fastener can be as lowas from about 20 to about 100 milligrams of a' high velocity detonatingexplosive and the loading of the priming charge is from about, 10

enlarged for clarity,fand

FIGURE 2 is a cross-sectional enlarged view of an explosively-releasablefastener having the ignition assembly of this invention, a stepped bore,and a bottom closure to facilitate explosive loading.

, In FIGURE 1, 1 designates the shell or housing member, e.g., a shank,a pin, a bolt, or a shaft, which mates, i .e., engages, with member 2,e.g., a nut, or otherwiseone of the frame elements to be connected, forexample, through screw threads. sociated element 2 can be attached toframe elements which can be of any desired configuration. The bore ofhousing member 1 contains a base charge 3 of a high velocity detonatingexplosive and a priming charge 5. Contiguous to priming charge 5 is the.contoured, two diameter plug 4 of an electrical insulating materialcomposed of head portion 4A and smaller diameter portion 4B. Nasher' 9,which is contiguous to head portion 4A and peripherally engages 4l3, isprovided with an enlarged bore adjacent the base of head portion 4Athereby forming internal electrode-receiving zone It The inner extremityof the plug is cavitied and an ignition charge 6 is housed in thiscavity. Electrode 7 extends into the plug from an external firingcircuit (not shown) and groundcontact electrode 7A is in contact with aconductive element in member 2. A bridgewire 8 of high electricalresistance connects the terminals of the electrical conductors withinthe plug and passes through the ignition charge contained in the plug sothat this bridgewire is in direct contact with the ignition composition.Member 2 is shown threaded to housing member 1, however, member 2 maytake the form of a plug screw threaded into 1 rather than the nut orseparate head shown in FIGURE l. The'threads joining the housing member1 to member 2 are set in a sealing compound, such as an epoxy cement,

at the time of assembly and the sealing composition is allowed toextrude into annulus 11 which further acts to support the'conductors.Due to the miniature size of this fastener, the electrical conductors 7are wrapped around the body of the plug subtencling an arc of about Thebridgewire is shown in the plane of the electrical conductors forconvenience, although in one embodiment it may go through the plug at anangle of about 45 to this plane and be'secured under the electricalconductors.

' However with a'larger unit, such Wrapping of the electrical 7 wirelength. At angles of greater than about the end of the inclinedconductor will be within the cavity for the ignition composition thusproviding an undesirable path for explosive gases to pass from thecavity to the exterior of the fastener head. Similarly, theground-contact electrode should be positioned outside and above theignition cavity.

InFiGURE 2 the elements are as in FIGURE 1, how- Both housing member 1and as- I ever, the explosively-releasable fastener is adapted for ease'in loading and has a stepped bore, i.e., the portion of the 'bore belowthe ignition assembly is of lesser diameter than about the base andpriming charges, and the priming charge 5 is spaced from the ignitioncharge 6 contained in the plug assembly. The fastener of this embodimentis loaded from the bottom and the base of the fastener is closed by aplug 12 of an extrudable composition such as lead and closure screw 13.Plug 12 retains base charge 3 in the bore of member 1 so that thethreads which mate with screw 13 can be cleaned to remove any explosivebefore the screw is inserted.

In operation, the electrical conductors 7 are connected in an electricalfiring circuit which may be either directly or remotely actuated. At thetime when actuation of the device is to be effected, an electric currentis applied to the electrical conductors. The passage of current throughthe high resistance bridgewire 8 produces heat which actuates theignition composition 6 and thus starts the explosive train resulting ininitiation of base charge 3. In applications wherein the ignitionassembly is used to initiate a base charge in the bore of a fastenerconnecting frame elements, which may be of any configuration, forcesexerted by the detonation or deflagration of the base charge, i.e.,detonation wave, shock fronts, gas pressure, and piling effects, aresufiicient to cause the housing 1 to be severed, due to weakening of theWall of the housing member of this section and the free spacesurrounding it. When the wall of the housing separates, the frameelements connected by the fastener are freed and, in re sponse toexternal forces acting thereon, e.g., gravity, mechanical forces,magnetic influences, the actual forces of detonation, etc., becomespatially separated. Owing to the fiuidand pressure-resistant sealformed about and by the contoured insulating plug, the frame elementattached to housing member 1 and to associated element 2 are not ventedto the atmosphere and, further, are not exposed to gases which may beproduced upon detonation of the base and priming charges.

The pressure exerted on the fastener head 2 is attenuated by the plugand this together with the mechanical baffiing of detonation gasesprovided by the contour of the plug prevents blow-out or leaking of thegases from the head of the fastener. The angular configuration of theelectrodes and the shielding provided by the body of the plug assuresthat they will be securely retained in the body of the plug and will notbe ejected from the fastener upon actuation of the base or breakingcharge.

Accordingly, the self-sealing fastener is severed at a predeterminedlocation (adjacent the base charge) by explosive means withoutdestroying or undermining the strength of adjacent structures andwithout producing shrapnel or in any way leaking in the presence of apressure differential at either end of the fastener either before orafter separation. This fastener does not require a separate closedbreech system or any other enlarged members which materially add to thecost, size, and weight of the fastener. The fastener is essentially nolarger than the actual fastening member, and any number of the units canbe separated in an essentially simultaneous action. The fastener isreleased with a minimum of vio-. lence and produces no shatteringeffects on the elements it connects.

The plug, which can be secured in the bore of the initiator or fastenershell by a waterand pressure-resistant sealant, e.g., a commercial epoxyresin, is constructed of a rigid, electrical insulating material whichis not reactive with the ignition and priming charge. Suitablestructural materials for this plug include thermosetting plastics orresinous materials such as, for example, Bakelite or epoxy resins andthermoplastics such as polyethylene, polypropylene, polyvinyl chloride,polytetrafiuoroethylene, acetal resins, and the like, and refractorymaterial such as silicon dioxide, e.g., porcelain or glass. When theignition assembly is to be used in an explosively releasable fastener asin the figures, the plug must be strong enough, e.g., by virtue of itsown structural strength and the reinforcement given by the sealingmeans, to withstand the forces exerted thereon when pressure builds upwithin the bore upon detonation of the base charge. This pressure limitsthe usefulness of thermoplastics since a given thermoplastic may bereactive under a particular set of temperature and pressure conditions.Therefore, the choice of insulating plug material for any givenapplication will be governed by the situation. The acetal resinmanufactured by E. I. du Pont de Nemours & Company under the trade nameDelrin, fulfilled the requirements of one embodiment of this invention.

The diameter of the electrodes used in the ignition assembly of thisinvention can be as small as 0.010 inch and generally will be about0.020 inch. Due to the unique assembly of the electrical element of highresistance, i.e., the bridgewire, which is provided by the plug, thisignition cavity may be as small as inch in diameter. The outer diameterswill be governed by the application in which the plug is being used. Thelength of the portion of the electrode 7 that extends into the plug maybe as short as /2 of the total height of the plug. The length of theinclined portion of the electrode will also be governed by theapplication, e.g., assemblies as shown in FIGURE 1 require somewhatlonger conductors than those needed with the FIGURE 2 assembly.

The walls of the ignition cavity need only be strong enough to withstandthe forces exerted in charging and assembling the units. In general, thewalls of the cavity may be from .015 to .030 inch thick. The ignitioncomposition can be loaded into the cavity at various magnitudes ofpressure. Usually, the pressure of compaction will exceed 10,000 p.s.i.Compaction pressures of 10,- 000 to 20,000 p.s.i. have been used withgood results, however, pressures as high as 100,000 p.s.i. can be used,if desired. The design of the ignition assembly of this invention isparticularly advantageous in loading of initiators since it is rigid andthe ignition charge can be separately loaded under controlled conditionsby a machine such as a rivet loader. Consequently, the initiator shellcan be loaded with the main and primer charges, the plug inserted intothe shell and pressed inwardly to subject the charges to the desiredpressure, preferably a pressure of above 10,000 p.s.i. The plug then issealed into the shell, e.g., by metal-filled epoxy resin, by crimping,or by swaging to confine the contents permanently under the appliedpressure.

The ignition composition preferably is of a pure compound such asessentially chemically pure lead azide, e.g., as available as RD1333lead azide, the double salt of cesium nitrate and cesiumdecahydrodecaborate, the doule salt of cesium nitrate and cesiumdodecahydrododecaborate, the multiple salt of basic lead picrate, leadpropionate and lead azide commercially available as Remington No. 8salt, or the lead salt of dinitro-o-cresol. If desired, a hinder or agraining agent may be used to aid in retaining the composition in thecavity. These compositions are initiated directly upon the passage ofelectricity through the resistance element with the accompanyingdissipation of heat and do not require the presence of conductiveparticles mixed Within the composition. Accordingly, there is no dangerof segregation of particles of the ignition composition during storagewhich would result in erratic firing response and the danger of pre-,

mature or accidental ignition .of the initiator by the passage ofspurious current through the composition is materially lessened.

The ignition assembly of this invention is conveniently prepared byinstalling the electrical conductors in the insulating plug head andwrapping them in arcs of about around the plug body. The bridgewire isinserted by pulling it up through axial slits provided in the ignitioncavity walls or, alternatively, is passed through perforations in thewalls, and thereafter the protruding ends are wrapped around theconductor legs and soldered or welded in place. The counterborein thetruncated conical insulating washer preferably is then coated withnon-conductive resin, e.g., an epoxy cement, and the washer is pressedover the plug body until it seats against the plug head and encloses theconductor legs. When the epoxy resin has cured, all electrical elementsare securely imbedded and virtually impervious to the eifects ofshock,vibration, or other disturbances.

As stated, the ignition assembly of this invention finds particularutility in actuating the explosive train in an explosively releasablefastener as shown in FIGURES 1 and 2. In such a fastener the elongatedshell 1 and associated member 2 can be constructed of a metal, e.g.,iron, steel, or other ferrous alloys, copper, bronze, aluminum,magnesium, etc., or of a polymeric material such as ofpolytetrafiuoroethylene or an acetal resin (Delr-in) having thestructural strength and rigidity to support the load between frameelements which the fastener or fasteners connect. When a polymerichousing is used, a contact plate or zone of an electrically conductivematerial is provided to act as a ground contract for the ground contactelectrode. The thickness and strength requirements of shell 1 andassociated element 2 naturally will depend upon the nature of theelements to be connected, i.e., the walls should have tensile strength,impact strength,

and shear strength necessary to withstand vectoral forces exerted by theobjects connected. The bore within the shank of the fastener preferablyis made at minimum expense of the fastener strength and provided at thelocation where separation is desired with sufficient quantity ofdetonating explosive to produce a dynamic pressurepulse of sufficientforce to overcome the strength of adjacent fastener walls, yetinsufficient to cause brisance injurious to the adjacent connectedstructures. If desired, the wall of the housing may be made thinner bymachining or 8 in FIGURE 2. Each plug was formed of Delrin acetal resinand was 0.11 inch thick, 0.089 inch in diameter over the charge cavityand 0.200 inch in diameter at the larger diameter head. The centralcylindrical cavity with 0.075 inch deep and 0.045 inch in diameter andwas filled with No. 8 Remington salt, a mixture of basic lead picrate,lead propionate, and lead azide, pressed at 10,000 psi. The

o-thenwise reducing the thickness of either the inside or 7 outside ofthe housing or the housing may be notched at the section at which theseparation is to be effected. Naturally the length of the housing willdepend upon the nature and size of charges to be used and the spacingdesired between frame elements.

The explosive employed as the base or breaking charge can be adeflagrating explosive; but, in orderto work the greatest economy insize while still retaining maximum fastening strength, the base chargeshould be an explosive which detonates at high velocity, namely avelocity greater than 1000 meters per second. While primary detonatingexplosives such as lead azide may be employed, preferably more powerfulexplosives, for example pentaerythritol tetranitrate (PETN),cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetr'anitramine(HMX), nitromannite, tetryl, or T T, explosives designated as secondaryexplosives, are used. As the priming charge, heatsensitive detonatingcompositions such as lead azide, diazodinitrophenol, or mercuryfulm-inate can be used. As mentioned heretofore, a quantity as low as 20millignams of high explosive in the fastener of this invention issufficient to sever the smaller fasteners at the predetermined sectionand does not cause undesirable violence and brisance. in a fastener of agiven size, quantities of high velocity detonating explosives somewhatlarger than the minimum breaking charge can be used, when centrallydisposed in a rigid housing, without loss'of the nonshatteringcharacteristics, but this necessitates an'increase in the thickness ofthe insulating plug to assure that a seal will be maintained after thedetonation occurs; and, since the main value of this invention it itssmall size, the practice is to utilize the minimum'explosive chargewhich will reliably break a given fastener. V

The following examples are presented as illustrative of the presentinvention but is not to be construed as limit- 7 ing the invention inany way.

xam ple I 7 Twelve ignition assemblies were prepared as describedelectrodes were of 26 ga., 7 x 34 stranded copper wire and thebridgewire was 0.001 in. diameter /20 platinum/ iridium (resistance 30).

Each of these ignition assemblies was inserted into the open extremityof an initiator shell (0.09 inch in diameter and 0.25 inch long)containing a base charge of 20 milligrams of HMX and a priming charge of15 milligrams of lead azide and sealed into place. The firingcharacteristics of the initiators were tested by connecting the centralelectrode to a condenser discharge blasting machine capable of producingfiring energies of to 62,000 ergs at voltages of from about 10 to about100 volts. The initiators were tested by attempting to discharge theoutput of the machine into the initiator by stepwise increasing thevoltages used to charge the condensers and recording the en- 'ergy atwhich the initiators functioned. The initiators were actuated reliablyand uniformly when 20,000 ergs were discharged from a 250 microfaradcapacitor. This level is well within the limits set forth by themilitary.

Example 2 Twelve faseners were prepared resembling that shown in FIGURE2. The housing 1 of each was 0.900 inch long, and its outer diameter was0.31 inch. The housing was threaded for essentially its entire lengthexcepting a inch segment adjacent to the base charge which was relievedto a diameter of 0.235 inch. The contoured 'bore (0.100 inch indiameter) within the housing contained 40 milligrams of HMX and 20milligrams of lead azide. The ignition assembly was as described inExample 1 except that in this case the plugs were fabricated of Bakeliteresin. One electrode extended into the housing from an external firingcircuit and the other electrical conductor was grounded to the elongatedhousing, w ich'was connected in the electric firing circuit.

The fasteners were fired by applying a direct current of approximately0.5 amper to the electrodes. The housing was severed at the sectionadjacent the base charge. Essent-ially no shrapnel was produced, andmicroscopic in spection and pressure studies of the fired fastenersrevealed that both sections of the fastener were sealed so that neitherdetonation gases nor electrical conductors would come in contact withobjects at the ends of the fastener and so that venting of gases to theoutside atmosphere would be precluded. T he severed fasteners werecapable of withstanding a hydrostatic pressure of at least 50 p.s.i.from either side of either fastener segment after detonation.

As will be evident to those skilled in the art, various modificationscan be made in the light of the foregoing disclosure and discussionwithout departing from the spirit and scope of the invention.

What is claimed is:

.1. An electric ignition assembly for use in an explosive initiatorwhich comprises:

(a) a plug of an electrical insulating material comprising (1) a bodyelement of essentially cylindrical crosssection having a head portionand a smaller diameter port-ion, a cavity having .an essentiallycylindrical wall being formed in the smaller diameter portion, saidcavity being concentric with the longitudinal axis of said body element,(2) a washer element of essentially truncated conical longitudinalcross-section contiguous to the base of said head portion and inperipheral engagement with the smaller diameter portion of said bodyelement, and (3) an internal electrode receiving zone bounded by saidbase, the periphery of said smaller diameter portion, and the bore ofsaid washer,

Qb) a first electrode extending through the head portion of the bodyelement of said plug and terminating in said electrode-receiving zone,

(c) a ground-cont act electrode spaced from said first electrode andextending from said zone through the head portion of the body element,the extremity of said ground-contact electrode Within said zone beingspaced from said cavity and spaced from the end of the first electrodein said zone by a distance at least equal to the radius of the smallerdiameter portion of the body element,

(d) an electrical element of high resistance bridging the extremities ofsaid electrodes within said Zone and passing through said cavity and thesmaller diameter portion of the body element, and

(e) a heat-sensitive ignition composition within said cavity and incontact with said bridgewire.

2. An explosively releasable fastener which is characterized by theabsence of shrapnel, explosive gases and ejected lead wire, and by itssealed condition after functioning, comprising an elongated housingmember adapted to engage With an associated element, said elongatedhousing having an axial bore closed at one end and containing insequence from said end (a) a base charge of high velocity detonatingexplosive;

(b) a priming charge contiguous to and in propagating relationship tosaid base charge;

(c) an ignition assembly in propagating relationship to said primingcharge, said ignition assembly being sealed in said bore by a water-andpressure-resistant sealant and comprising (1) a plug of an electricalinsulating material comprising (a) a body element of essentiallycylindrical cross-section having a head portion and a smaller diameterportion, a cavity having an essentially cylindrical wall being formed inthe smaller diameter portion, said cavity being concentric with thelongitudinal axis of said body element, (b) a washer element ofessentially truncated conical longitudinal cross-section contiguous tothe base of said head portion and in peripheral engagement with thesmaller diameter portion of said body element, and (c') an internalelectrode-receiving zone bounded by said base, the periphery of saidsmaller diameter portion, and the bore of said washer,

(2) a first electrode extending into the head portion of the bodyelement, the axis of said electrode at its point of entry into the plugbeing coincident with the longitudinal axis of said plug, said electrodehaving an inclined portion formed integrally therewith extendingradially from said axis thereby defining an angle of declination withthe portion of said electrode extending into the plug, said inclinedportion be- 19 ginning within the head portion of the body element andterminating in said electrode-re ceiving zone,

(3) a ground-contact electrode extending from said zone through the headportion of the body element essentially parallel to but radiallydisplaced from the longitudinal axis of the plug, the extremity of saidground contact electrode within said zone being spaced from said cavityand spaced from the end of the first electrode in said zone by adistance at least equal to the radius of the smaller diameter portion ofthe body element,

(4) an electrical element of high resistance bridging the extremities ofsaid electrodes within said zone and passing through said cavity and thesmaller diameter portion of the body element, and

(5) a heat-sensitive ignition composition within said cavity and incontact with said bridgewire; and

(d) a rigid member secured to the extremity of said housing contiguousto said ignition assembly and reducing the diameter of the axial bore insaid elongated housing, said first electrode of the ignition assemblyextending through said member.

3. A releasable fastener as in claim 2 wherein the fastener is made ofmild steel and loading of the base charge is from about 20 to aboutmilligrams and the loading of the priming charge is from about 10 toabout 50 milligrams of a primary explosive.

4. A releasable fastener as in claim 2 wherein the elongated fastenerhousing is of mild steel and the major diameter of the bore of theelongated housing is about A to about /2 the diameter of the housing atthe section of the housing to be severed.

5. An ignition assembly as in claim 1 wherein said first electrodeenters the head portion of the plug on the plugs longitudinal axis.

6. A releasable fastener as in claim 2 wherein said axial bore is ofsmaller diameter between the ignition assembly and priming charge thanit is about said priming charge and the base charge, and the ignitioncomposition and priming charge are spaced from each other by a distanceless than the gap sensitivity value of the priming charge.

References Cited by the Examiner UNITED STATES PATENTS 2,556,672 6/51Bergh et al. 89-1.5 XR 2,815,698 12/57 Burrows 89- 1 X 2,842,059 7/58Plumley et a1. l02--28 2,920,569 1/60 Menke et a1. 102-28 X BENJAMIN A.BORCHELT, Primary Examiner. SAMUEL W. ENGLE, Examiner.

2. AN EXPLOSIVELY RELEASABLE FASTERNER WHICH IS CHARACTERIZED BY THEABSENCE OF SHRAPNEL,EXPLOSIVE GASES AND EJECTED LEAD WIRE, AND BY ITSSEALED CONDITION AFTER FUNCTIONING, COMPRISING AN ELONGATED HOUSINGMEMBER ADAPTED TO ENGAGE WITH AN ASSOCIATED ELEMENT, SAID ELONGATEDHOUSING HAVING AN AXIAL BORE CLOSED AT ONE END AND CONTAINING INSEQUENCE FROM SAID END (A) A BASE CHARGE OF HIGH VELOCITY DETONATINGEXPLOSIVE; (B) A PRIMING CHARGE CONTIGUOUS TO AND IN PROPAGATINGRELATIONSHIP TO SAID BASE CHARGE; (C) AN IGNITION ASSEMBLY INPROPAGATING RELATIONSHIP TO SAID PRIMING CHARGE, SAID IGNITION ASSEMBLYBEING SEALED IN SAID BORE BY A WATER-AND PRESSURE-RESISTANT SEALANT ANDCOMPRISING (1) A PLUG OF AN ELECTRICAL INSULATING MATERIAL COMPRISING(A'') A BODY ELEMENT OF ESSENTIALLY CYLINDRICAL CROSS-SECTION HVING AHEAD PORTION AND A SMALLER DIAMETER PORTION, A CAVITY HAVING ANESSENTIALLY CYLINDRICAL WALL BEING FORMED IN THE SMALLER DIAMETERPORTION, SAID CAVITY BEING CONCENTRIC WITH THE LONGITUDINAL AXIS OF SAIDBODY ELEMENT, (B'') A WASHER ELEMENT OF ESSENTIALLY TRUNCATED CONICALLONGITUDINAL CROSS-SECTION CONTIGUOUS TO THE BASE OF SAI D HEAD PORTIONAND IN PERIPHERAL ENGAGEMENT WITH TEH SMALLER DIAMETER PORTION OF SAIDBODY ELEMENT, AND (C'') AN INTERNAL ELECTRODE-RECEIVING ZONE BOUNDED BYSAID BASE, THE PERIPHERY OF SAID SMALLER DIAMETER PORTION, AND THE BOREOF SAID WASHER, (2) A FIRST ELECTRODE EXTENDING INT THE HEAD PORTION OFTHE BODY ELEMENT, THE AXIS OF SAID ELECTRODE AT ITS POINT OF ENTRY INTOTHE PLUG BEING COINCIDENT WITH THE LONGITUDINAL AXIS OF SAID PLUG, SAIDELECTRODE HAVING AN INCLINED PORITON FORMED INTEGRALLY THEREWITHEXTENDING RADIALLY FROM SAID AXIS THEREBY DEFINING AN ANGLE OFDECLINATION WITH THE PORTION OF SAID ELECTRODE EXTENDING INTO THE PLUG,SAID INCLINED PORTION BE-